I've had my 86 Cat 27 for about 9 months now. It's on the hard now and I removed the batteries to the basement. Two Group 34 AGM new in '09. One battery is holding 12.6v. The other can't seem to get up much past 6.75v. I have a very good and new dual batterycharger on the boat. At home I have a single 6 amp charger with a 8 amp boost. When I plug in the single and try to boost it it immediately switches to Float. Even if I leave it plugged in overnight it won't get over 6.75 volts. Is this battery FUBR? During the season I normally had the shore power plugged in and the dual charger on. Everything was normally off but I left one of the batteries 'On' to run the auto bilge pump. So maybe, If in rained a great deal, The pump might run a few minutes and switch off. I usually trie to switch between batteries on which would be left on.
I've read enough about batteries to know that one is suppose to replace both batteries, not just one. And not to mix types. Two AGM batteries would be very expensive and really frustrating knowing that one battery is just fine. So, the question is, are these 'rules' all that iron clad? Does anyone have any experience replacing just one? Mixing types? Also, is there a way to bring back the low battery?

I've had my 86 Cat 27 for about 9 months now. It's on the hard now and I removed the batteries to the basement. Two Group 34 AGM new in '09. One battery is holding 12.6v. The other can't seem to get up much past 6.75v.

Most AGM's should hold close to 13 v on resting voltage. So 6.75 volt is either an uncharged AGM battery, or a really dead 12 volt FLA, or a fully partially charged 6 volt AGM, maybe even a 6 volt FLA.

It's likely that you have a bad meter, don't know how to read a meter, or both bats are in Q's

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I have a very good and new dual battery charger on the boat. At home I have a single 6 amp charger with a 8 amp boost. When I plug in the single and try to boost it it immediately switches to Float. Even if I leave it plugged in overnight it won't get over 6.75 volts.

That would imply a very sulfated battery, or the wrong charging voltage.

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Is this battery FUBR? During the season I normally had the shore power plugged in and the dual charger on. Everything was normally off but I left one of the batteries 'On' to run the auto bilge pump. So maybe, If in rained a great deal, The pump might run a few minutes and switch off. I usually trie to switch between batteries on which would be left on.

As usual, human intervention, can't quit do what machine intervention was designed to do.

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I've read enough about batteries to know that one is suppose to replace both batteries, not just one. And not to mix types.

Ok

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Two AGM batteries would be very expensive

REALLY

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and really frustrating knowing that one battery is just fine.

Is one of the bat's really just fine?

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So, the question is, are these 'rules' all that iron clad? Does anyone have any experience replacing just one? Mixing types? Also, is there a way to bring back the low battery?

if the batteries are separate and on a 1-2 switch replacing one isn't a bit deal.

if they are permineatly joined together as one battery bank then it's more important.

also I'm betting when you move the switch back and forth, the bilge pump is still always on the same battery. as it should be wired before the switch. but if the boat is left on shore power and the charger is on. this shouldn't matter.

Most AGM's should hold close to 13 v on resting voltage. So 6.75 volt is either a fully charged AGM battery, or a over charged 12 volt FLA, or a fully charged 6 volt AGM, maybe even a 6 volt FLA.

It's likely that you have a bad meter, don't know how to read a meter, or both bats are in Q's.

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I have a very good and new dual battery charger on the boat. At home I have a single 6 amp charger with a 8 amp boost. When I plug in the single and try to boost it it immediately switches to Float. Even if I leave it plugged in overnight it won't get over 6.75 volts.

That would imply a very sulfated battery, or the wrong charging voltage.

As usual, human intervention, can't quite do what machine intervention was designed to do.

Your battery definitely needs replacing and if I were you I would replace both. Four years out of AGMs on a boat is doing pretty good. I would replace with either golf cart batteries or LiFePo4 batteries.